Preheat the oven to 200C. Scatter the aubergines in a large roasting tin and drizzle over the oil; season with salt and toss to coat thoroughly. Roast for 15 minutes, stir, then roast for a further 15 minutes.
While the aubergines cook, finely chop the onion. Heat a drizzle of oil in a saucepan, add the onion and chilli flakes, and fry gently for 8 minutes. Tip in the tomatoes and cook gently for 10 minutes (covered with a lid). Stir occasionally. Stir in the aubergines when ready and keep on a low heat until ready to serve.
To make the polenta, pour the stock and milk into a large saucepan and bring to the boil over a high heat. Reduce to a simmer and gradually add the polenta stirring continually to avoid lumps. Once all added, simmer for 4 minutes, stirring occasionally. Use a long-handlled spoon to avoid burns.
Remove polenta from the heat, season with an extra pinch of salt, and stir in the butter and parmesan. Serve the polenta in warm bowls with the aubergine sauce ladled on top and a scattering of fresh parsley.